The mammalian heartbeat is stimulated by electric signals within the heart muscle. The heart usually beats in a steady rhythm. However, cardiac arrhythmias are common abnormalities in the rhythm of the heart beat where the heart may beat very fast (“tachycardia”) or very slow (“bradycardia”). Arrthymias may arise when electrical impulses in the heart's electrical network travel irregular pathways or repeat the same pathways over and over.
Arrhythmias are often associated with problems in micro or macro anatomical zones in heart tissue. Arrhythmias may result from the development of an improper electrical circuit caused by a developmental anomaly or a myocardial scar. Treatment sometimes involves altering those tissues. The destruction of these tissues and any other abnormal electrical circuits is called ablation therapy.
Ablation is a procedure to restore normal rhythm by destroying very small, carefully selected parts of the heart that cause arrhythmias. Ablation can be performed either by catheters or during heart surgery.
One method of ablation is radiofrequency (“RF”) catheter ablation, in which RF current is applied to the treatment zone via a blood vessel or directly on the inner or outer surface of the heart. RF ablation came into clinical practice at the end of the 1980's and has revolutionized the treatment of patients with abnormal heart rhythms.
In RF ablation, necrosis or scar tissue in pathologic tissue is induced, thus reorienting the electrical transmission in the heart and alleviating the arrhythmia. In this technique, a site of abnormal impulse formation or a critical part of the arrhythmic pathway may be located, for example, by cardiac activation mapping during the arrhythmia. RF energy can be applied to the area, resulting in the destruction of a small portion of critical tissue and alleviation of the arrhythmia for the patient.
In RF catheter ablation, a clinician (typically an electrophysiologist) may introduce a cardiac catheter into the chambers of the heart via the peripheral vasculature. An electrode is mounted on the end of the catheter through which RF current from an RF generator is passed with proper grounding and siting of the catheter. The passage of current most often produces a localized lesion that alters electrical pathways but does not affect cardiac function.
However, application of RF techniques is not without complication or contraindications in certain patient populations. As some examples, catheter ablation of some arrhythmias may require application of RF current in the vicinity of coronary arteries. Application of the current generates heat, which can damage adjacent blood vessels. This inherently increases the risk of damage to the coronary arteries due to the proximity of the coronary arteries to the ablation sites of accessory pathways. With RF ablations performed in the vicinity of coronary arteries, susceptibility to damage is inversely proportional to vessel diameter. It is also known to those of ordinary skill in the art that damage to the coronary arteries can occur from ablation performed with energy sources other than radiofrequency current, including without limitation, microwave, ultrasound, or high intensity focused ultrasound (“HIFU”) energy.
Moreover, obstruction of the right coronary artery as a late sequela after RF application to the tricuspid annulus has been previously described in an animal model. It has also been inferred that catheter ablation in humans targeting accessory pathways in the right free wall may damage the right coronary artery. In similar fashion, stenosis of the coronary artery is a substantial risk of RF current application in children and although rare, catheter ablation of atrial flutter has also been associated with acute thrombotic occlusion of the right coronary artery. A risk of damage to the coronary arteries remains in some regions, as one example only, in the ostium of the middle cardiac vein, and in children.
Thus, an unmet need exists for methods of controlling or alleviating cardiac arrhythmias while decreasing the risks of complications from the application of RF current or other sources of ablative energy.